Method of manufacturing membrane-electrode assembly for fuel cell

ABSTRACT

The present invention provides a method of manufacturing a membrane-electrode assembly for a fuel cell, in which a binder is spray-coated on a surface of a polymer film, a catalyst slurry is bar-coated on a surface of an electrolyte membrane, bonded on the binder, to form a catalyst electrode layer, a bonded assembly of the electrolyte membrane and the catalyst electrode layer is separated from the polymer film, after a drying process, to obtain a 2-layer MEA, and the thus obtained 2-layer MEAs are used to form a 3-layer MEA or a 5-layer MEA by a hot pressing process. Accordingly, the present methods solve the problems associated with prior art that the loss of catalyst is considerable, since the catalyst slurry is directly spray-coated on the membrane, and the catalyst electrode layer in a solid phase is hot-pressed on both surfaces of the membrane.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(a) on KoreanPatent Application No. 10-2007-0090741, filed on Sep. 7, 2007, theentire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to a method of manufacturing amembrane-electrode assembly (MEA) for a fuel cell. More particularly,the present invention relates to a method of manufacturing a 3-layer MEAin which a catalyst electrode layer is stacked on both surfaces of amembrane, and a method of manufacturing a 5-layer MEA in which gasdiffusion layers (GDL) are stacked on both surfaces of a membrane withcatalyst electrode layers disposed thereon.

(b) Background Art

In general, fuel cells are devices that convert the chemical energy offuel directly into electrical energy by an electrochemical reaction offuel and oxygen in air without combustion. Such fuel cells haveattracted much attention as a zero emission power generation system andcan be applied to the supply of electrical power for small-sizedelectrical/electronic devices, especially, portable devices, as well asto the supply of electrical power for industry, home, and vehicle.

A polymer electrolyte membrane fuel cell (PEMFC) has advantages ofhigher output density, faster response, and simpler systemconfiguration, and thus extensive research for using the PEMFC as avehicle power source or a stationary power generation system hascontinued to progress.

The PEMFC includes a membrane-electrode assembly (MEA), which is a majorcomponent and positioned at the most inner portion. The MEA includes ananode, a cathode and an electrolyte membrane disposed therebetween.

Referring to FIG. 1, catalyst electrode layers 3, i.e., the anode andthe cathode are formed by uniformly coating a desired amount of catalyston both surfaces of a polymer electrolyte membrane 4. Gas diffusionlayers (GDLs) 2 are positioned at the outside of the MEA, i.e., on thesurfaces where the catalyst is present, and separators 1 having flowfields for supplying fuel and exhausting water produced by the reactionare positioned at the outside of the GDL 2.

In general, a unit cell of the PEMFC includes one polymer electrolytemembrane, two catalyst electrode layers, two GDLs, and two separators. Aplurality of such unit cells are stacked to form a fuel cell stack of adesired scale.

In the MEA, an oxidation reaction of hydrogen occurs at the anode of thefuel cell to produce hydrogen ions and electrons. The thus producedhydrogen ions and electrons are transferred to the cathode through thepolymer electrolyte membrane and a conducting wire, respectively.

At the same time, a reduction reaction of oxygen occurs at the cathodereceiving the hydrogen ions and electrons from the anode to producewater. Here, electrical energy is generated by the flow of the electronsthrough the conducting wire and by the flow of the protons through thepolymer electrolyte membrane.

The MEAs are generally classified into a 3-layer MEA and a 5-layer MEA.The 3-layer MEA includes an electrolyte membrane and catalyst electrodelayers disposed thereon. The 5-layer MEA includes an electrolytemembrane, catalyst electrode layers and gas diffusion layers.Conventionally, the 5-layer MEA is formed by bonding gas diffusionelectrodes (GDEs) to both sides of the electrolyte membrane using a hotpressing process.

It is known that the performance of 3-layer MEA is better than that ofthe 5-layer MEA under the same conditions. The reason for this is thatthe interfacial resistance between the electrolyte membrane and thecatalyst electrode layer of the 3-layer MEA is lower than that of the5-layer MEA.

The interfacial resistance between the electrolyte membrane and thecatalyst electrode layer has a direct effect on the MEA performance, andthe MEA performance has a direct effect on the fuel cell performance.Accordingly, it is understood that, if the interfacial resistance of theMEA is reduced, it is possible to improve the fuel cell performanceremarkably.

A typical method of manufacturing a 3-layer MEA is catalyst-coatedmembrane (CCM) method in which catalyst is coated on an electrolytemembrane. A typical method of manufacturing a 5-layer MEA iscatalyst-coated GDL (CCG) method in which catalyst is coated on GDLs toform an anode and a cathode and the catalyst-coated GDL is bonded to apolymer electrolyte membrane.

In more detail, the 3-layer MEA is manufactured in such a manner that acatalyst slurry of low concentration is directly spray-coated on theelectrolyte membrane using a spray gun to form a catalyst electrodelayer, or the catalyst slurry is coated on a film and then subjected toa decal method. In the decal method, a polymer film as a release filmcoated with the catalyst slurry is hot-pressed onto a membrane such thatan electrode is transferred onto the membrane.

The 3-layer MEA manufactured by directly spray-coating the catalyst onthe membrane has an advantage that it is possible to minimize theinterfacial resistance between the catalyst electrode layer and themembrane; however, since the catalyst slurry of low concentration shouldbe spray-coated repeatedly on the membrane, it takes a long processingtime and there is inevitably a loss of the expensive catalyst in termsof the spray process characteristics.

Moreover, it is not easy to directly spray-coat the catalyst on themembrane, and the coating method is limited only to the spray coatingmethod. The reason for this is that, if the membrane is in directcontact with a solvent contained in the catalyst slurry, the membrane isswollen and deformed. Thus, it is difficult to directly spray-coat thecatalyst on the membrane by other methods.

In the event that the catalyst is directly spray-coated on the surfaceof the membrane using the spray coating method, the solvent should beremoved repeatedly after the direct spray-coating in order to reduce theamount of solvent coming in contact with the membrane, prevent thecatalyst electrode layer from being cracked, and minimize thedeformation of the membrane.

In the event that other methods are employed, since a large amount ofsolvent present in the catalyst slurry is in direct contact with themembrane, the membrane is deformed, the catalyst electrode layer is notformed uniformly, and thus it is impossible to manufacture the MEA.

For such reasons, the following method is generally used. That is, thecatalyst is coated on a release film, in which no deformation is causedby the solvent, using a spray, a screen printing, or a casting knife anddried at a high temperature. Then, the resulting release film is placedon the membrane and pressed at a high temperature and pressure, thustransferring the catalyst electrode layer coated on the release film tothe membrane.

Such a method is called a decal method. The decal method has anadvantage in that the release film may not be deformed by the solventsince the catalyst electrode layer is coated on the release film.However, since it employs the release film, the manufacturing cost isincreased and it additionally requires the pressing process, comparedwith the method of direct spray-coating the catalyst on the membrane.

Moreover, the decal method has another disadvantage in that, since thecatalyst electrode layer in a solid phase, from which the solvent isremoved, is transferred to the membrane, the contact area between thecatalyst electrode layer and the membrane is reduced, and theinterfacial resistance is increased, compared with the direct coatingmethod.

FIG. 2 is a schematic diagram illustrating a conventional process offorming a catalyst slurry on a release film. As shown in the figure, acatalyst slurry 13 is coated on a polymer film 11 as a release filmusing a casting knife in the shape of a bar or an applicator 12. Sincethe above components have a flat film surface and a micro-scale gap, thecatalyst slurry 13 is passed through the gaps and coated by thethickness of the gap, thus forming a catalyst electrode layer 13′ havinga small thickness on the polymer film 11.

Such a method of coating the catalyst slurry on the film surface iscalled a casting or bar coating method.

FIG. 3 is a schematic diagram illustrating a conventional hot pressingprocess, in which a release film 11 including a catalyst electrode layer13′ is compressed on both surfaces of a membrane 14, respectively, byapplying heat to transfer the catalyst electrode layers 13′ on themembrane 14, and FIG. 4 is a diagram illustrating a configuration of a3-layer MEA manufactured by the decal method.

According to the conventional MEA manufacturing methods using the barcoating and decal methods of FIGS. 2 to 4, the catalyst slurry is placedon the expensive polymer film 11 such as polyethyleneimine (PEI),silicon coated polyethyleneterephthalate (PET), andpolytetrafluoroethylene (PTFE) coating film and, then, the applicator 12is moved in the casting direction to coat the catalyst slurry 13 on thepolymer film 11 in a constant thickness.

At this time, the thickness of the coated catalyst slurry 13 can beadjusted by the applicator 12.

After the catalyst slurry is coated, the resulting polymer film 11 isdried in an oven at 60° C. to 100° C. for 30 minutes to completelyremove the solvent contained in the catalyst slurry 13. The driedpolymer film 11 is placed on both surfaces of the membrane 14 andsubjected to the hot pressing process at 100° C. to 150° C. for 1 to 5minutes, thus forming a 3-layer MEA (from which the polymer film is tobe removed).

In the thus formed 3-layer MEA of FIG. 4, since the catalyst electrodelayer 13′ in a solid phase, from which the solvent is completelyremoved, is transferred to the membrane 14, the interfacial adhesionbetween the catalyst electrode layer 13′ and the membrane 14 is reducedcompared with the direct coating method, and the manufacturing cost isincreased due to the use of expensive release film.

Meanwhile, the 5-layer MEA is manufactured in such a manner that anelectrolyte membrane is disposed between two gas diffusion electrodes(GDEs) and pressed at a high temperature and high pressure, in whicheach of the GDEs is prepared by directly coating the catalyst on a GDL.

The thus formed 5-layer MEA has an advantage of a simpler manufacturingprocess; however, since the catalyst electrode layer and the membrane inthe same solid phase are bonded to each other by the hot pressingprocess, the contact area between the catalyst electrode layer and themembrane is reduced and thus the interfacial resistance is increasedmore than that of the 3-layer MEA.

Moreover, it has a further drawback in that the method of coating thecatalyst slurry on the GDL is limited to the spray coating method inwhich the loss of catalyst is considerable.

The information disclosed in this Background section is only forenhancement of understanding of the background of the invention andshould not be taken as an acknowledgement or any form of suggestion thatthis information forms the prior art that is already known to a personskilled in the art.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to solve the aboveproblems, and an object of the present invention is to provide a methodof manufacturing an MEA for a fuel cell, which can provide a simplerprocess, reduce the loss of catalyst, provide an MEA with improvedperformance, and reduce the manufacturing time and cost.

In one aspect, the present invention provides a method of manufacturinga membrane-electrode assembly for a fuel cell, the method comprising:spray-coating a binder on a surface of a polymer film; placing anelectrolyte membrane on the binder-coated polymer film and then heatingand compressing them to bond the electrolyte membrane to the polymerfilm by the binder; coating a catalyst slurry on a surface of theelectrolyte membrane to form a catalyst electrode layer; separating abonded assembly of the electrolyte membrane and the catalyst electrodelayer from the polymer film, after a drying process, to obtain a 2-layermembrane-electrode assembly; and stacking two of the 2-layermembrane-electrode assemblies such that the electrolyte membranesthereof face each other and then heating and compressing them to form a3-layer membrane-electrode assembly.

In another aspect, the present invention provides a method ofmanufacturing a membrane-electrode assembly for a fuel cell, the methodcomprising: spray-coating a binder on a surface of a polymer film;placing an electrolyte membrane on the binder-coated polymer film andthen heating and compressing them to bond the electrolyte membrane tothe polymer film by the binder; coating a catalyst slurry on a surfaceof the electrolyte membrane to form a catalyst electrode layer;separating a bonded assembly of the electrolyte membrane and thecatalyst electrode layer from the polymer film, after a drying process,to obtain a 2-layer membrane-electrode assembly; and disposing the2-layer membrane-electrode assembly between a gas diffusion layer and agas diffusion electrode prepared by stacking a catalyst electrode layeron a surface of another gas diffusion layer and then heating andcompressing them to form a 5-layer membrane-electrode assembly.

Preferably, the polymer film is formed of a material selected from thegroup consisting of polyethyleneterephthalate, polyvinylchloride,polyimide, poly(aryl-ether-ether-ketone), polyurethane, polysulfone, andpolytetrafluoroethylene.

Suitably, the binder is formed of a material selected from the groupconsisting of perfluorosulfonic acid, polytetrafluoroethylene,perfluorocarboxylic acid, sulfonated poly(aryl-ether-ether-ketone),sulfonated polysulfone, and sulfonated polyimide.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like.

Other aspects of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a general configuration of a unit cellof a fuel cell;

FIG. 2 is a schematic diagram illustrating a conventional process offorming a catalyst slurry on a release film;

FIG. 3 is a schematic diagram illustrating a conventional hot pressingprocess, in which a release film including a catalyst electrode layer iscompressed on both surfaces of an electrode membrane, respectively, byapplying heat to transfer the catalyst electrode layers on the electrodemembrane;

FIG. 4 is a diagram illustrating a configuration of a 3-layer MEAmanufactured by a decal method;

FIGS. 5 and 6 are flow diagrams illustrating a process of manufacturinga 2-layer MEA in which an electrolyte membrane is bonded to a catalystelectrode layer in a manufacturing method in accordance with a preferredembodiment of the present invention;

FIG. 7 is a schematic diagram illustrating a process of manufacturing a3-layer MEA in accordance with a preferred embodiment of the presentinvention;

FIG. 8 is a schematic diagram illustrating a process of manufacturing a5-layer MEA in accordance with a preferred embodiment of the presentinvention;

FIG. 9 is an SEM photograph of the surface of a 2-layer MEA manufacturedby a bar coating method in accordance with the present invention; and

FIG. 10 is an SEM photograph of the section of the 2-layer MEAmanufactured by a bar coating method in accordance with the presentinvention.

Reference numerals set forth in the Drawings includes reference to thefollowing elements as further discussed below:

11: release film 12: applicator 13: catalyst slurry 13′ catalystelectrode layer 14: electrolyte membrane 15: polymer film 16: spray gun17: binder 18: 2-layer MEA 21: GDL 22: GDE

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the drawingsattached hereinafter, wherein like reference numerals refer to likeelements throughout. The embodiments are described below so as toexplain the present invention by referring to the figures.

FIGS. 5 and 6 are flow diagrams illustrating a process of manufacturinga 2-layer MEA in which an electrolyte membrane is bonded to a catalystelectrode layer in accordance with a preferred embodiment of the presentinvention.

First, as shown in FIG. 5, a binder 17 is spray-coated on the surface ofa polymer film 15 using a spray gun 16.

Suitably, the polymer film 15 is formed of a material selected from thegroup consisting of polyethyleneterephthalate, polyvinylchloride,polyimide, poly(aryl-ether-ether-ketone), polyurethane, polysulfone, andpolytetrafluoroethylene (trade name: Teflon).

Preferably, the binder 17 is formed of a material selected from thegroup consisting of perfluorosulfonic acid (trade name: Nafion),polytetrafluoroethylene, perfluorocarboxylic acid, sulfonatedpoly(aryl-ether-ether-ketone), sulfonated polysulfone, and sulfonatedpolyimide.

Next, as shown in FIG. 6B, an electrolyte membrane 14 is placed on thepolymer film 15 with the binder 17 coated thereon and then subjected toa hot pressing process at about 100° C. to 150° C. for about 1 to 5minutes, thus forming a film-membrane assembly in which the polymer film15 is bonded to the electrolyte membrane 14 by the binder 17.

Subsequently, as shown in FIGS. 6C and 6D, a catalyst electrode layer13′ is formed by casting (bar-coating) a catalyst slurry 13 on thesurface of the electrolyte membrane 14 of the thus formed film-membraneassembly.

That is, the catalyst slurry 13 is coated on the surface of theelectrolyte membrane 14 using a casting knife or an applicator 12. Here,since the above components have a flat film surface and a micro-scalegap, the catalyst slurry 13 is passed through the gaps and coated by thethickness of the gap, thus forming the catalyst electrode layer 13′having a small thickness on the surface of the electrolyte membrane 14.

In this case, the catalyst slurry 13 is coated on the electrolytemembrane 14 in a constant thickness when the applicator 12 is moved inthe casting direction in a state where the catalyst slurry 13 is placedon the surface of the electrolyte membrane 14, and the thickness of thecoated catalyst slurry 13 can be adjusted by the applicator 12.

According to the conventional method, when the catalyst is coated on thesurface of the membrane using the applicator, the membrane is in contactwith the solvent contained in the catalyst to be swollen and wrinkled,and thus the catalyst comes off the membrane. However, according to thepresent invention, since the polymer film 15 and the electrolytemembrane 14 are bonded to each other by the binder 17, it is possible toprevent the electrolyte membrane 14 from being swollen and wrinkled bythe solvent of the catalyst slurry 13.

Next, the film-membrane assembly in which the polymer film 15 and theelectrolyte membrane 14 are bonded to each other is placed in an ovenand dried at about 60° C. to 100° C. for about 1 to 5 minutes and, asshown in FIG. 6E, the electrolyte membrane 14 and the catalyst electrodelayer 13′ are separated from the polymer film 15, thus forming a 2-layerMEA.

FIG. 9 is an SEM photograph of the surface of the 2-layer MEAmanufactured by a bar coating method in accordance with the presentinvention, and FIG. 10 is an SEM photograph of the section of the2-layer MEA.

Using the thus formed 2-layered MEA, a 3-layer MEA or a 5-layer MEA canbe manufactured. FIG. 7 is a schematic diagram illustrating a process ofmanufacturing a 3-layer MEA, and FIG. 8 is a schematic diagramillustrating a process of manufacturing a 5-layer MEA.

First, according to the process of manufacturing a 3-layer MEA, as shownin FIG. 7, two 2-layer MEAs 18 are stacked such that the electrolytemembranes 14 face each other and subjected to a hot pressing process tobond the two electrolyte membranes 14.

At this time, the hot pressing process is carried out at about 100° C.to 150° C. for about 1 to 5 minutes in a state where the two 2-layerMEAs 18 are stacked, thus forming a 3-layer MEA in which the catalystelectrode layer 13′ is formed on both surfaces of the electrolytemembrane 14.

According to the process of manufacturing a 5-layer MEA, as shown inFIG. 8, the 5-layer MEA is manufactured using a GDL 21, a 2-layer MEA18, and a GDE 22 in which a catalyst electrode layer 13′ is formed onthe surface of another GDL 21. That is, the 2-layer MEA formed bythe-above described method is disposed between the GDL 21 and the GDE 22and then subjected to a hot pressing process to be bonded to each other.

The hot pressing process is carried out at about 100° C. to 150° C. forabout 1 to 5 minutes in a state where the 2-layer MEA 18 is disposedbetween the GDL 21 and the GDE 22 such that the catalyst electrode layer13′ of the GDE 22 is bonded to the electrolyte membrane 14 of the2-layer MEA 18, thus forming a 5-layer MEA, in which the GDL 21, thecatalyst electrode layer 13′, the electrolyte membrane 14, the catalystelectrode layer 13′ and the GDL 21 are stacked and fixed in thesequential order.

The GDE 22 is formed by spray-coating the catalyst slurry on one surfaceof the GDL 21 through the conventional method. At this time, althoughthe catalyst slurry is inevitably spray-coated on the GDL 21, thecatalyst slurry is not spray-coated on the electrolyte membrane, andthus it is possible to reduce the amount of catalyst slurry remarkablyin the overall process, compared with the conventional method ofmanufacturing a 5-layer MEA.

Moreover, while the catalyst electrode layer in a solid phase is bondedto both surfaces of the electrolyte membrane by a hot pressing processin the conventional 5-layer MEA manufacturing process, the catalystelectrode layer 13′ of the GDE 22 is bonded to one surface of theelectrolyte membrane 14 by the hot pressing process (the catalystelectrode layer on the other surface of the electrolyte membrane 14 isbar-coated) in the present invention, and thus it is possible toincrease the overall contact area between the catalyst electrode layer13′ and the electrolyte membrane 14 and reduce the interfacialresistance remarkably compared with the conventional 5-layer MEA.

As described above, according to the method of manufacturing amembrane-electrode assembly for a fuel cell, a binder is spray-coated onthe surface of a polymer film, a catalyst slurry is bar-coated on thesurface of an electrolyte membrane, bonded on the binder, to form acatalyst electrode layer, a bonded assembly of the electrolyte membraneand the catalyst electrode layer is separated from the polymer film,after a drying process, to obtain a 2-layer MEA, and the thus obtained2-layer MEAs are used to form a 3-layer MEA or a 5-layer MEA by a hotpressing process.

Accordingly, the present invention provides the following effects:

1) It is possible to minimize the interfacial resistance between thecatalyst electrode layer and the electrolyte membrane and reduce themanufacturing time, the loss of catalyst and the manufacturing cost,compared with the conventional 3-layer MEA manufacturing process, inwhich the catalyst slurry is spray-coated repeatedly, it takes a longprocessing time, and the loss of expensive catalyst is considerable;

2) Since the present invention does not employ the release film, i.e.,the expensive polymer film used in the decal method, it is possible toreduce the manufacturing cost;

3) While the conventional 5-layer MEA manufacturing process employs twoGDLs spray-coated with the catalyst slurry, the present inventionemploys one GDL that requires a spray coating, and thus it is possibleto reduce the amount of catalyst slurry remarkably in the overallprocess; and

4) While the catalyst electrode layer in a solid phase is bonded to bothsurfaces of the electrolyte membrane by a hot pressing process in theconventional 5-layer MEA manufacturing process, the catalyst electrodelayer of the GDE is bonded to one surface of the electrolyte membrane bythe hot pressing process (the catalyst electrode layer on the othersurface of the electrolyte membrane is bar-coated) in the presentinvention, and thus it is possible to increase the overall contact areabetween the catalyst electrode layer and the electrolyte membrane andreduce the interfacial resistance remarkably compared with theconventional 5-layer MEA.

The invention has been described in detail with reference to preferredembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

1. A method of manufacturing a membrane-electrode assembly for a fuelcell, the method comprising: spray-coating a binder on a surface of apolymer film; placing an electrolyte membrane on the binder-coatedpolymer film and then heating and compressing them to bond theelectrolyte membrane to the polymer film by the binder; coating acatalyst slurry on a surface of the electrolyte membrane to form acatalyst electrode layer; separating a bonded assembly of theelectrolyte membrane and the catalyst electrode layer from the polymerfilm, after a drying process, to obtain a 2-layer membrane-electrodeassembly; and stacking two of the 2-layer membrane-electrode assembliessuch that the electrolyte membranes thereof face each other and thenheating and compressing them to form a 3-layer membrane-electrodeassembly.
 2. The method of claim 1, wherein the polymer film is formedof a material selected from the group consisting ofpolyethyleneterephthalate, polyvinylchloride, polyimide,poly(aryl-ether-ether-ketone), polyurethane, polysulfone, andpolytetrafluoroethylene.
 3. The method of claim 1, wherein the binder isformed of a material selected from the group consisting ofperfluorosulfonic acid, polytetrafluoroethylene, perfluorocarboxylicacid, sulfonated poly(aryl-ether-ether-ketone), sulfonated polysulfone,and sulfonated polyimide.
 4. A method of manufacturing amembrane-electrode assembly for a fuel cell, the method comprising:spray-coating a binder on a surface of a polymer film; placing anelectrolyte membrane on the binder-coated polymer film and, then,heating and compressing them to bond the electrolyte membrane to thepolymer film by the binder; coating a catalyst slurry on a surface ofthe electrolyte membrane to form a catalyst electrode layer; separatinga bonded assembly of the electrolyte membrane and the catalyst electrodelayer from the polymer film, after a drying process, to obtain a 2-layermembrane-electrode assembly; and disposing the 2-layermembrane-electrode assembly between a gas diffusion layer and a gasdiffusion electrode prepared by stacking a catalyst electrode layer on asurface of another gas diffusion layer and then heating and compressingthem to form a 5-layer membrane-electrode assembly.
 5. The method ofclaim 4, wherein the polymer film is formed of a material selected fromthe group consisting of polyethyleneterephthalate, polyvinylchloride,polyimide, poly(aryl-ether-ether-ketone), polyurethane, polysulfone, andpolytetrafluoroethylene.
 6. The method of claim 4, wherein the binder isformed of a material selected from the group consisting ofperfluorosulfonic acid, polytetrafluoroethylene, perfluorocarboxylicacid, sulfonated poly(aryl-ether-ether-ketone), sulfonated polysulfone,and sulfonated polyimide.